1. Field of the Invention
This invention is concerned with a wireline tool for cutting, retrieving and separating retrieved sidewall core samples from a borehole.
2. Discussion of Related Art
Core samples are plugs of the native rock cut from the rock formations at depth levels of interest. The structure, composition and texture of the rock formations as evidenced by the core samples are of quantitative analytical interest to miners, civil engineers, petrophysicists, geologists, oceanographers and other earth scientists.
Boreholes penetrating the earth are drilled for many different purposes, such as water wells, oil and gas wells, brine recovery wells, and foundation studies. Ordinarily, the drilling process particulates and contaminates the rock formations that are penetrated by the drill so that the drill cuttings themselves are of limited use analytically. For that reason, special tools have been developed for cutting cores from the living rock at selected depth levels, often in the borehole sidewall after the borehole has been drilled.
In days of yore, a plurality of hollow punch-core bits were loaded in barrels mounted in an elongated coring tool. The bits are aimed perpendicularly to the borehole sidewall and distributed at selected intervals longitudinally along a mandrel one or two tens of feet long. Each of the bits was secured to the mandrel by a short flexible cable. The mandrel comprising the coring tool was lowered to a desired depth from a wireline whereupon explosive charges were triggered behind the respective core bits, driving them into the sidewall. A core sample was retained in the hollow bit after detonation. The coring tool was then recovered from the borehole after breaking loose from the sidewall, the plurality of punch-core bits and heir contained core samples. Verification of the core sequence was assured because each core was resident in a physically separate core bin which was secured to the coring tool by the attached cable so that the relative depth of each core in the sequence was positively known. A missing core was readily detectable by simple inspection.
There were problems with explosively-driven punch-coring tools, not the least of which was the need for use of explosives, a hazardous proposition.
A different type of wireline coring tool is in use that employs a hollow rotary coring bit that is mounted in a housing fitted in the mandrel of a down-hole wireline coring tool. The housing comprises an assembly including means for laterally extending a rotating cutting bit into, and retracting the bit from a borehole sidewall. Upon retraction, following the core-cutting operation, the bit is rotated 90.degree. whereupon a push rod shoves the cut core out of the bit into a storage tube. The storage tube is mounted longitudinally with respect to the mandrel, beneath the housing containing the cutting bit. A typical sidewall core is about an inch in diameter and about one-and-one half to two inches long. After the first core is cut, the coring tool is moved up the borehole to a new location where another core is cut and stored in the core storage tube. The process is repeated until the storage tube is full, perhaps acquiring twenty or more cores per downhole trip.
One such arrangement is taught in U.S. patent application Ser. No. 08/146,441, filed Oct. 29, 1993 in the name of Jacques Maissa et al., assigned to the assignee of this invention, and now U.S. Pat. No. 5,411,106, issued May 2, 1995, which is incorporated herein by reference.
It is preferable that the respective cores residing in the storage tube be physically separated from one another. Additionally, completion of a particular sidewall coring operation does not necessarily result in successful recovery of a core. Therefore the core sequence must be properly indexed such the non-existence of one or more cores can be positively verified.
U.S. Pat. No. 5,310,013, issued May 10, 1994 to A. C. Kishino et al. discloses means for placing an indexing marker above each recovered core. No indication of a missing core appears to be provided for.
U.S. Pat. No. 4,714,119, issued Dec. 22, 1987 to Joel Hebert et al. also provides means for inserting a marker disc between the recovered cores. Both of the above patents employ a tubular reservoir of marker discs that are urged upwards by a pusher spring. A separate tubular core storage bin is mounted beside to marker reservoir. After a core is deposited in the core storage bin, a system of levers shoves a marker disc laterally from the open upper end of the marker reservoir to the mouth of the storage bin. Somehow a magnetic solenoid at the top of the storage bin, captures the marker disc (which is magnetic) so that it will fall on top of the previously-deposited core. The mechanical arrangement of the two references is deemed to be far too complicated to be practical.
U.S. Pat. No. 4,449,593, issued May 22, 1984, to Gary D. Bruce et al. also teaches use of indexing washers to separate core samples. Here again, Bruce's system involves a separate storage bin for the marker discs and a lever system for shoving the marker into the core storage bin.
In all three of the above patent references, the marker disc must in some way fall into place rather than be firmly deposited in place.
There is a need for a mechanically simple, reliable means for positively dispensing core a separator between individual cores during a core storage operation.